Highly sensitive radioimmunoassays have been developed to estimate microgram/ml levels of both IgE and IgG classes of specific antibody response in the sera of patients with filariasis and other helminth infections. Immune responses to various antigens from each of the major stages of the parasites are also being analyzed in an effort to understand the pathogenesis of the wide spectrum of clinical presentations seen in these infections. The antigens derived from each of the stages are being rigorously characterized both biochemically and immunochemically. Furthermore, the basis of cross reactivity among various nematodes and cestodes is also being studied by inhibition radioimmunoassays with a goal toward developing more specific immunodiagnostic reagents.